This invention relates generally to video time-base correctors of the ype having an internal sync generator for generating a composite blanking signal according to television standards which specify which lines in a video signal are normally blanked and which lines are normally not. This invention further relates to selectively unblanking normally blanked lines in the vertical blanking interval of a video signal, such that specially inserted data on these lines may be retrieved.
A broadcast television signal is composed of video data and reference synchronizing signals. The video data represents what a viewer perceives as the video picture, while the reference synchronizing signals are used by the television receiver to organize the video data to properly form the television picture. A moving video picture as seen by the viewer is actually a series of still pictures called video frames. A frame is a single snapshot of the moving video picture. The video data in the video frame is organized as a number of horizontal lines.
In a television picture tube, an electron beam selectively scans each line of video data horizontally across the face of the picture tube. One complete picture, or frame of video is actually composed of two interlaced scans down the screen. Starting at the top of the screen, a line of video data is scanned and then the second line below it is scanned, skipping a line. This means that when a field is scanned across a picture tube, only every other line is actually scanned. For example on the first field, line 1 would be scanned and then line 3, then line 5, and so on. For the second field, line 2 would be scanned and then line 4, and so on.
Thus each video frame consists of typically two fields of video data which are interlaced to reduce flicker. During the field sync period and for several lines thereafter the video data is normally blanked before commencement of the active picture period. The period between the end of active video on one field and the beginning of active video on the next field is known as the field blanking period. During this time, in the picture generating apparatus, such as a camera, and likewise the receiver, vertical retrace occurs. The lines in the vertical blanking period following the post-equalizing pulses may be used to carry additional video data, not necessarily related to the active video. Such video data might include vertical interval test signals (VITS), vertical interval time code (VITC), teletext data, or closed caption data for the hearing impaired. It is desirable that any additional video data added to the composite video signal during the vertical blanking period can be selectively blanked or unblanked on a line-by-line basis as well as a field-by-field basis, since the two vertical blanking periods may contain different data.
There are a number of horizontal lines that occur during the vertical blanking interval which, by definition are normally blanked. It is these horizontal lines which occur during the vertical blanking period that are the subject of this disclosure. The composite blanking signal is a combination of the horizontal and vertical blanking signals.
In the broadcast environment, video signals may be received from many sources and some of the signals may have additional information encoded in the lines of video data of the vertical blanking period as described above. It is desirable for the operator to be able to selectively decide which of the lines containing special data signals during the vertical blanking period ot unblank so that they may be passed on. For example, if the first three lines of the vertical blanking period after the post equalization pulses of each field of a video frame contain VITS, which is used only in the studio and thus not transmitted to viewers, and the video lines 18 and 280 (for M/NTSC) of fields 1 and 2 respectively, contain closed captioned information, which must be transmitted with the broadcast signal, and the signal is to be sent to the broadcasting tower, the video engineer would like to be able to leave the VITS lines normally blanked, but would also like to unblank the closed captioned lines so that they will be sent to the home decoder of the hearing impaired.
Another desirable feature would be the ability to remotely control the unblanking operation. For example, while reproducing a signal from a video tape recorder it would be desirable for the engineer to be able to turn on or off the unblanking process from the video tape recorder control panel.
The composite blanking signal is created as one of the standard outputs of a reference signal by a reference sync generator. Reference sync generators can be found in video equipment such as video cameras and video time-base correctors and provide blanking signals which form a part of the composite video signal. Unblanking of normally blanked lines of the vertical blanking period can be done by any device that creates new reference signals by means of a reference sync generator. Such unblanking is usually performed as an additional feature of a time-base corrector.
A time-base corrector is a device that receives a video signal from a source such as a video tape recorder and produces an output video signal that has been synchronized to a standard reference signal and has been corrected for a variety of errors. The errors include drop-outs and tape induced timing errors caused by tape stretch. As part of this process, a new set of reference signals, including the composite blanking signal, is created by an internal reference sync generator. Because of this fact, the time-base corrector is an excellent location for the implementation of unblanking operations.
One known method of controlling the unblanking of lines in the vertical blanking period is to vary the vertical blanking duration. This method is imprecise and can only be used to unblank consecutive lines at the end of the vertical blanking period.
Additional problems for an unblanking system are created by the fact that there exist four major television standards used worldwide and the lack of standardization concerning which lines of the vertical blanking period should be used for special information. In an unblanking system the number of lines per frame of video, the position of the vertical blanking period, the number of lines in the blanking period, and the composite blanking signal are all critical in its design. These number change for each standard. The standard used in the United States and Japan is referred to as NTSC, while the standard used in Great Britain is call PAL. There are additional standards such as PAL-M and SECAM. These standards differ in how the video signal is arranged, such as the number of lines per frame (NTSC & PAL-M=525, PAL & SECAM=625). The number of lines in the vertical blanking period also vary from standard to standard, and even within a standard.
As a result of these problems there is a need for a method or apparatus that can provide selective unblanking of any combination of lines during the vertical blanking period. There is also a need for a common method or apparatus that will work with any video standard. Such a method or apparatus desirably also allows for remote control of the unblanking operation.